1. Field of the Invention
The present invention relates to hydrophobic aramid fibers and to a process for making them.
During the manufacture of aramid fibers, a processing finish is often used. For certain applications, for example, in impregnation of the fibers or a woven product, processing finish which is still present on the fibers must be removed. Thereafter, fluoro-containing compounds as water repellent agent are applied to the fibers in the woven product.
The nature of the fiber-resin interface in aramid composites is especially important. It is also known that the aramid yarns used, for example, in filament wound structures such as pressure vessels or in hard ballistic protective armours, such as helmets, perform better when the fiber-to-resin adhesion level is well controlled.
2. Description of the Prior Art
UK Patent Application No. 2,221,928, published Feb. 21, 1990 discloses treatment of textile materials with ketene dimers to impart water repellant properties thereto. The textile materials of the publication are primarily wool, cotton, and polyester/cotton blends. The textile materials treated in that publication are always dried and are never disclosed to be solvent- or water-swollen.
However, the finishes according to this reference are not suitable for the purposes of the present invention in terms of surface frictional properties, hydrophobic and use of the .resulting fibers in resin composites. The above-mentioned reference does not yield fibers having the desired properties and which are ready to be used.
Japanese Patent Application Kokai 60-258 245 relates to an aqueous dispersion containing a ketene dimer, a cationic acrylamide polymer, and an anionic dispersing agent, for the treatment of cellulose fiber textiles in order to generate softness and water-repellency.
Therefore, it is one object of the present invention to provide a ready to use tailored engineered surface energy aramid fiber for applications where reduced or no water dynamic sorption and well controlled and low fiber surface energy are needed. Ready to use means that the aramid fiber is not subjected to further treatment such as removal of the processing finish, applying of a water repellent or adjusting the fiber to resin level for composite application.
It is another object of the present invention to offer a process by way of which the ready to use fiber is produced. Continuous (in-line) and batch-wise (off-line) processes for producing the modified aromatic polyamide fibrous material have to be provided.
Another object of the present invention is to provide an aramid fibrous material, useful for reinforcing rubber and composite articles or other polymeric matrices (epoxy, polyester, phenolic polymers), for materials which involve in their production a twisting, knitting, braiding, spiralling or weaving operation.
Another object of this invention is to provide a highly processable aramid element (yarn, thread, cord, staple, pulp, short fibers) usable as a reinforcing element for elastomeric and traditional composites. The improved processability of this product leads to higher performance of the final system (for example, higher strength conversion in fabrics).
Another object of the invention is to provide aramid fibers which can be used without twisting in production lines which involve for example a knitting or weaving operation of a single yarn. When used in a twisted form, for example in a cord, the tenacity and modulus of the aramid element is better utilized in the final cord structure than with commercially available products.
It has surprisingly been found that by the treatment of never-dried aramid fibers by means of a surface reactant, the processability, hydrophobicity and the fiber-to-resin adhesion level are improved. At the same time, toxicological risks by the use of formerly used fluorine-containing water repellents at high temperature as well as the potential bond degradation by the use of formerly used silicone oils for the control of the fiber-to-resin level are obviated.
According to this invention, the application of certain surface reactants for reaction on the surface of never-dried aramid fibers provides a new surface enhanced fiber which exhibits not only excellent processability characteristics with respect to friction, but also a completely hydrophobic surface. The resultant fibers exhibit a low wickability and an enhanced glycol resistance, the latter being extremely important for e.g. composite materials in automobile radiator systems. The fibers also show reduced discoloration kinetics if exposed to daylight. The hardness of the bobbins (package density) of the fibers according to the invention is also significantly improved. The use of the surface reactant further obviates additional steps of treatment by which the fiber-to-resin adhesion level is controlled for the application as a reinforcing element for composite applications or obviates additional scouring and fluoro-treatment of fabric woven structures. Fabrics made, of the tailored surface energy fiber of this invention exhibit higher wearing comfort due to increased air permeability and vapor transport.
The end use performance of the final system is consequently significantly improved.